Single-mode optical fibers can support two polarization modes. If the core of a single-mode fiber is perfectly circular, the two polarization modes propagate with the same speed. However, due to manufacturing tolerance, the core of the fiber varies slightly from a perfect circle thereby causing the two polarizations modes to propagate at slightly different speed and causing polarization mode dispersion. Polarization mode dispersion (PMD) is a major problem in high bit-rate data optical transmissions due to the two polarization modes propagating at different group velocities, which in turn generates signal distortion. The difference in group velocity, arise from small residual birefringence due to fiber asymmetries or stress, either internal or externally applied. Both of internal stress and external perturbations vary with environmental conditions, such as the temperature along the link, and have a stochastic behavior. Signal degradation occurs when the time delay (DGD, i.e. Differential Group Delay) between the Principal States of Polarization (PSP), the fastest and the slowest polarization modes, is a large fraction of the bit slot, typically more than 10%, and the input State Of Polarization (SOP) equally excites both PSP. PMD is typically wavelength dependent, so that each channel of a WDM multichannel transmission in an optical fiber suffers different signal degradation.
Polarization dependent loss (PDL) or polarization dependent gain (PDG) also affects the quality of an optical transmission because they also vary stochastically when concatenated in a fiber link. Hence, PDL and PDG cause signals to have different amplitude according to their polarization state and degrade a receiver performance having a fixed decision threshold.
Due to the above there is a need for methods and arrangements enabling improved PMD mitigation in optical transmission systems.